a. trade
Some trades were more likely to bring a worker into contact with asbestos than others. Obviously, a 'lagger' - a worker whose role it was to install or remove insulation around pipes and in boilers - was most likely to be exposed to airborne asbestos fibres. Depending on the type of insulation product being handled, the process of either installing or removing insulation could liberate a higher or lower amount of fibre. If, for example, KLite blocks were being installed on pipes with bends and angles or obstacles to negotiate, the calcium silicate blocks had to be cut at an angle for fitting purposes. Cutting was done with handsaws or mechanical saws. That process created a lot of dust which contained a percentage of amosite asbestos fibre.
A different trade - say, an instrument maker or a trade assistant to an instrument maker - may only have intermittently encountered an atmosphere with asbestos fibre, if at all, depending on whether asbestos insulation needed to be removed in order for an instrument to be installed or removed. It would also depend on whether that instrument maker was present when or soon after the asbestos insulation was disturbed.
Similarly, a cleaner or a rigger might be heavily exposed to asbestos fibre if working in an environment where insulation (say blocks or sections) had been cut leaving debris that could still be disturbed - even, in some cases, drifting down onto workers below. Or they may only be intermittently or rarely exposed to asbestos fibre if not working in and around insulation installation or removal areas or in periods of time when asbestos insulation product was not being disturbed.
But, the incidence of exposure for many of the trades working amongst those who worked more directly with asbestos fibre would be difficult to predict. It could readily be said that they might on occasions have been exposed to asbestos fibre: not that they must have been or, at least, not regularly.
The most comprehensive resource for information about a particular trade, and the nature and likely degree of exposure to airborne asbestos fibre associated with that trade, was supplied through Mr Drewett's OEAs for each claimant. As described in further detail below, Mr Drewett's statement, incorporating his OEAs, was permitted to be tendered subject to relatively few objections.
b. direct or indirect exposure
The degree of likelihood that a person's asbestos disease can be attributed to Hardie-BI asbestos, as distinct from other manufacturer's product, may be influenced by whether inferred exposure to Hardie-BI product was direct or indirect.
As previously explained, direct exposure is the inhalation of asbestos fibre in the industrial environment where the asbestos insulation is situated. Indirect exposure occurs through a worker, who him or herself being directly exposed, carries the asbestos fibre on clothing, boots etc into the home thus causing household members to come into contact with the dust which they inhale.
Common sense would suggest that the quantity fibre to which a household member may be indirectly exposed will generally be less than that of the directly exposed worker. But the extent of a household member's potential exposure will itself be impacted by the number of workers in the household who worked at an SECV facility; the trades that those workers worked in; the places each different worker worked in; the duration of time over which the workers carried the dust into the home; the particular tasks the household member had with the clothing; and the extent to which the person's asbestos condition can be sensibly explained by exposure to any other source, and so on.
c. period
Where the supply and construction records for the particular facility indicated the probable installation of Hardie-BI product from a specific point in time, it is obviously relevant to know whether the claimant was present at the facility thereafter, or only before. It is also relevant to know the duration of a claimant's work at a particular facility at which Hardie-BI fibre can be inferred to have been installed. Clearly, the longer a claimant worked at a facility, the higher the degree of probability that he or she encountered airborne asbestos fibre liberated from Hardie-BI product.
But it is also highly relevant to know whether the period of the worker's presence at a facility was during a construction phase or only a maintenance phase. It stands to reason that the disturbance of asbestos fibre into the atmosphere was likely to have occurred in greater volume when insulation was being installed in bulk, rather than removed and replaced on a piece-meal basis.
d. type of asbestos insulation product
It may be of relevance to know whether the claimant worked with, or was likely to have worked with, different kinds of asbestos insulation product. Products that were cut; that were sprayed on as dry fibre; or which required mixing from dry powder form with water to make cement - were each more likely to cause exposure to airborne fibre than, say, uncut sheets or boards. Likewise, asbestos insulation made from an amphibole fibre had a greater causal potency than of the non-amphibole type.
e. exposure to asbestos fibre from other manufacturers
In assessing the probability that a particular claimant's illness was caused by exposure to Hardie-BI product, it would seem logical to know the worker's exposure over time to asbestos fibre from all sources. Logically, it would seem to be more difficult to infer the necessary causal role of Hardie-BI fibre in respect of a worker exposed to asbestos fibre from many different sources over a long period of time than in respect of a worker who was only ever exposed to fibre from Hardie-BI product. But, that statement itself is somewhat controversial in relation to 'indivisible' injuries.
f. nature of illness
In terms of the attribution of causal significance to Hardie-BI product in respect of a given worker's illness, the nature of that illness may be of importance. Bearing in mind Professor Henderson's opinion that, with the malignant forms of illness, all exposure (above background) is causative, exposure to asbestos of other origin is not necessarily so significant. However, for non-malignant forms of illness the same principle does not apply so that it is at least relevant in respect of those illnesses to evaluate the relative degrees of exposure to all product to form a view of causation (ie liability).
But even if liability can be assumed, the question of the degree of responsibility of the Hardie-BI partners for a worker's damage consequent upon an illness, vis-à-vis other contributors, may require an assessment of the relative amounts of exposure. Whether that is so for both divisible and indivisible illnesses remains a live issue.
g. interval between exposure and onset of illness
A necessary ingredient for finding a causal relationship between an identified (or inferred) exposure to Hardie-BI fibre and a worker's illness, is the existence of a minimum gap in time before onset of the illness. An onset that occurs too close to the posited exposure event will not be attributed to that event because of the known latency period between exposure and the manifestation of illness in relation to different illnesses. Professor Henderson has examined each claimant against that criterion.[62]